Halogen incandescent lamps, for example of the type described in the referenced U.S. Pat. No. 4,876,482, Stadler, often use a cross support element, typically of quartz glass, to retain the current lead-ins and support wires, if desired. The referenced European Patent Application 0 173, 995, Westlund et al, describes a single-ended lamp whcih does not use such a cross bar. The filament is retained only by a support wire which is melted-in together with the connecting foils in a pinch or press seal of the lamp. A lamp of this type, when designed for power network voltage, requires a filament having a relatively high resistance, and consequently of substantial length. The filament, to obtain this length, is bent into two luminescent arms, so that the general shape of the filament is approximately inverted-V shaped or inverted-U shaped.
It has been found, in operation, that the concept of the design of the aforementioned European patent application has some disadvantages. These lamps are sensitive to shocks and vibrations. If the lamp is subjected to a shock while it is lit, the two arms of the filament can touch each other. The portions of the filament which still have current flowing therethrough, thus burn out due to overloading. This is due to the application of the full network voltage over then only a fraction of the length of the filament. Usually, the engagement is about midway between the length of the filament legs, so that only half the normal filament resistance is present, causing burn-out and hence lamp failure.
If the lamp is operated in a horizontal position, the filament, due to sagging upon heating, can touch the interior of the bulb wall. The free length of the filament arms is comparatively long. Some hang-through or sag of the filament cannot be avoided; a typical hang-through is about 10%. Any contact of the filament with the lamp bulb causes blackening thereof and furthermore reduces the lifetime of the lamp.
Various types of holders for the filament have been proposed. For example, the referenced German Utility Model 19 52 467, Schmidt, describes a halogen incandescent lamp, particularly suitable for photographic use, which has a holder for a generally U-shaped filament, formed by a glass rod or glass tube which has at the end remote from the base one or more hooks to hold the filament melted therein. The filament itself is short, since a focussable light source is to be provided. The danger of engagement of the filament portions against each other upon shock or vibration, therefore, is low. The glass tube or glass rod is introduced into the pinch seal of the bulb and so positioned that a portion thereof is within the pinch seal, and coupled to the respective supply leads from the filament. Such a construction, at first glance, may also be suitable for general service halogen incandescent lamps. Yet, the requirements placed on service lamps for general illumination differ substantially from those of photo flood lamps, particularly with respect to lifetime and rated power. Price, also, is a factor. In photo lamps, the stability of the filaments is higher due to the usually greater diameter of the filament wires. Photo lamps, also, can be handled carefully since they are used only in specific controlled environments.
Glass rods or glass tubes located within halogen incandescent lamps of the type suitable in the photo lamp field hve the tendency to blacken during the desired lifetime of a general service lamp, which is in the order of about 2000 hours. Further, due to the high temperature, for the long period of time, the tubes may devitrify. Blackening and devitrification indirectly lead to premature lamp failure. In photo lamps, which have a lifetime in the order of about 50 hours, these effects are of minor importance.
Lamps constructed in accordance with the aforementioned referenced U.S. Pat. No. 4,876,482, Stadler, operate well; yet, assembly of the mount, together with the cross bar, is costly, since a number of individual elements must be assembled and properly positioned. The lamps, also, are relatively long.
U.S. Pat. No. 3,840,953 is another illustration of a halogen incandescent lamp utilizing a cross element or bridge of quartz into which the lead-in wires for the filament are melt-connected, together with an auxiliary support wire, of tungsten, to support the filament intermediate its length.
Lamps using an exceedingly long filament, such as lumiline lamps, also known as T or tubular lamps, require support along the length of the elongated filament. European Patent 0 143 917, Blumberg et al, assigned to the assignee of the present application, illustrates a lamp of this type having an elongated axial light emitting filament. If this lamp is operated horizontally, the long filament may sag through and touch the inner wall of the elongated tubular bulb. To prevent that, a plurality of spiral or ring-shaped filament support wires are provided, engaging the inner wall of the bulb and supporting the filament intermediate its length. The bulb wall is formed with small depressions to retain the wire holders for the filament.
Assembly of such lumiline bulb together with the filament holding wire spiral is complex and expensive, particularly if the spirals have to be snapped into their holding projections, formed internally by depressing the outer bulb at selected positions. For a standard lumiline lamp, about four such holders are required, which increases the cost of materials and assembly. Manufacture of a filament with holders therefor also is complex and expensive. To prevent damage to the filament, for example by deformation, the filaments msut be separated from one another, then the holder rings applied thereto, and separately handled and assembled. The further manufacture of the lamp, thus, becomes uneconomical and cannot be easily automated.